Radar System Engineering

272 ANTENNAS, SCANNERS, A.VD STABILIZA TIO.Y [SEC. 92
spicuous trend toward shorter wavelengths which has characterized radar
ever since its earliest development.
In the course o developing and testing a new antenna, the distribution
of the energy in the beam may be expressed by means of a polar
diagram or antenna pattern. Figure 9.1 displays such a pattern and
FIG. 9 1.—A typical antennapattern showingthe main beamof width 61and the sidelobes.
shows that the beamwidth e is the full beamwidth at half power. A
small amount of po’wer is unavoidably radiated in undesired directions,
forming the “side lobes’) shown in this figure.
92. Round and Cut Paraboloid Antemas.-One very common type
of microwave radar antenna takes the form of a paraboloidal reflector 1
with a source of radiation, or “antenna feed, ” at its focus. Since the
wavelength of the radiation is short in comparison
with the dimensions of the reflector, it
/+’
is profitable in an introductory discussion to
regard the operation of the antenna as a problem
in ray optics (Fig. 9.2). Figure 9.2 does
not explain the side lobes or the beamwidth,
but it does serve to emphasize the fact that
the feed is always so designed as to be directive.
Figure 9.3 represents an automatic record of
the energy radiated at various angles from a
well-designed feed, the directional power being
displayed as a function of angle. It is obvious
that nearly all of the energy from the feed will
FIG. 9.2.—RaY diagram of a strike a properly placed paraboloid, which will
paraboloid antenna.
then collimate the radiation. Figure 9.4
shows the radiation from an incorrectly designed feed. A correct design,
used in the 3-cm band, is illustrated in Fig. 9.5, which shows two dipoles
excited by the radiation field from the ~- by l-in. waveguide transmission
line. The dipoles are so adjusted in length and position that their combined
effect is to direct the radiation back within a cone surrounding the
line without reflecting radiation into the waveguide.
Equation (1) requires further comment in the case of a paraboloid
reflector of which the perimeter has been so trimmed (“ cut paraboloid”)
that its width and height are unequal. If the beam is pointed horizon-
I The subject of paraboloid and other types of antennas is developed in Microwave
Ankmno Theory and Design, Vol. 12 of this series.